In the processing of semiconductor products of the type used for very large scale integrated (VLSI) circuit devices, to either image or cure the photoresist material for the very small geometries is difficult for several reasons. One of the essential requirements of a photoresist is its ability to withstand high temperatures while retaining control of both edge profile and line dimensions. It is known that, in order to fabricate VLSI devices, reactive ion etching (RIE) in a plasma chamber is essential to reproduce the photoresist geometry required. Sometimes the temperature in the plasma chamber can be so high that it causes the photoresist material to flow, if not to burn. Moreover, the choices of photoresist materials and the materials to be etched are very limited when RIE is to be performed. Accordingly, high temperatures and tougher resist materials are required for RIE for curing and stabilization procedures.
Photoresist material sensitive to deep ultraviolet (DUV) exposure is being used more widely in this art to overcome the problems of RIE outlined above. For example, poly(methylmethacrylate) (PMMA) is being widely used in conjunction with diazo-type photoresists, such as Kodak 809 or Shipley AZ 1350J positive photoresist material. They are used to provide so-called double-layer or multi-layer photoresists for certain advantages in the curing and imaging steps. See the article by B. J. Lin entitled, "Multi-Layer Resist Systems as a Means to Submicron Optical Lithography," IDEM 1982, pages 391-394, and the article by B. F. Griffing entitled, "An Operational Two Level Photoresist Technology," IDEM 1981, pages 562-565, for detailed descriptions of such multi-layer resist systems.
There are known apparatus for improving the thermal properties of photoresists, such as PRIST (Photo Resist Image Stability Technique), described by W. H. L. Ma, "Plasma Resist Image Stabilization Technique (PRIST) Update," Proc. Submicron Lithography, vol. 333, pp. 19-23, SPIE, Bellingham, Wash., 1982, or DUV treatment of photoresists followed by a high temperature bake as described by R. Allen, M. Foster, Y. T. Yen, "Deep U.V. Hardening of Posistive Photoresist Patterns," J. Electrochemical Soc., vol. 129, pp. 1379-1381, 1982. However, the disadvantage of the PRIST system is that the process needs a plasma etcher which is very complex and costly. Moreover, it is very difficult to install production photoresist stablization equipment for such processes. The Fusion Corporation of Rockville, Md. provides a DUV apparatus that utilizes microwave energy to excite a mercury vapor lamp. However, the disadvantage of this DUV approach is that the apparatus is bulky and generates a lot of heat.
In the art of flood exposure, for either curing a photoresist material or exposing the photoresist material, as for multi-level pattern image processing, the illumination apparatus must not overheat the resist material. In spite of the efforts, for example, as outlined above, there is a need in the art for apparatus for flood exposing photoresist material rapidly and with good resolution either for imaging purposes or for curing the material without damaging it.
Griffing, cited above, for example, describes a DUV blanket exposure equipment using a cadmium (Cd) arc lamp. Since the radiation from a Cd arc lamp is not collimated, Griffing describes the use of a baffle structure to provide some collimation of the beam by absorbing the stray radiation that is not substantially collimated. A cylindrical reflector is used to guide the radiation from the source to the wafers. Griffing describes that the critical limitation of the cut-off angle of approximately 20.degree. was required, however, to effect imaging 1.5 micrometers thick PMMA photoresist for developing 0.5 micrometer lines.